A drag issue : to bungee wings or not to bungee

[lord.harshil]

There have been various discussions about this I'm sure but I found this on the web and felt like sharing it.

To those who feel offended by this, tough luck



From: Bill Lais
Date: Wed, 11 Mar 1998 14:56:06 EST
To: rickhave@juno.com, cavers@www.geek.com

Subject: Re: Bungee Wings: A Real Drag?

In a message dated 98-03-11 14:42:13 EST, you write:

Recently, myself and several engineering colleagues were performing wind
tunnel tests on a new airfoil, under design within our division at
Grumman Aerospace. Somehow the subject of technical diving and buoyancy
compensation systems came up. We got to talking about the drag profiles
on certain manufacturers brands of BCs and one of the guys suggested that
we run some actual aerodynamics tests. A couple of the engineers are big
time mixed gas wreck divers with lots of experience on deep wrecks such
as the Doria and Republic. They were touting the benefits of the OMS
bungeed wings and I was arguing that the Dive Rite wings provided a
vastly improved knife edge profile with far less drag.

A simple surface area calculation showed that the bungeed wings had an
area of over 12 square feet inflated. This is over 4 times the area of
the Dive Rite wings. The huge box like profile caused immense drag and
horrendous turbulence when we observed it in the wind tunnel. The bungee
cords caused massive vorticial eddies all along the wings. It was very
interesting to observe the colored smoke trails streaming in chaotic
cyclonic patterns around the brightly colored bungees. It was almost
psychedelic. These results were in stark contrast to the Dive Rite wings
which showed very little turbulence and almost hydrodynamic behavior such
that the wings probably generate lift when there is current flow across
them. Needless to say, the extra inflator assembly on the OMS wings
created marked disturbances in the laminar flow field of the trace smoke.
When the wings were in the collapsed state the turbulent patterns were
even more spectacular, clearly discrediting the myth that the bungees
reduce drag by supposedly decreasing the surface area.

After the tests there were no more arguments except about what other uses
these wings might be put to. One of the guys suggested we employ them as
portable cushions for homeless people, another suggestion involved using
them as toxic waste storage containers.

There was much discussion and speculation about the personality and
physical characteristics of the rocket scientist who had designed these
wings. I suspect that the same individual carries a fire extinguisher in
the front seat of his car along with the spare gas can.

Rick

Taken from frogkick.dk, this is NOT my own work.

 

[TSandM]

Interesting but not surprising results.

Bungied wings are really a solution for an unnecessary problem.

 

[AndyNZ]

Not that I am a fan of bungied wings myself, but having both designed both ships hulls and aeroplane wings in my time... there is a huge difference between aerodynamics and hydrodynamics....!

 

[danvolker]

There have been various discussions about this I'm sure but I found this on the web and felt like sharing it.

To those who feel offended by this, tough luck



From: Bill Lais
Date: Wed, 11 Mar 1998 14:56:06 EST
To: rickhave@juno.com, cavers@www.geek.com

Subject: Re: Bungee Wings: A Real Drag?

In a message dated 98-03-11 14:42:13 EST, you write:

Recently, myself and several engineering colleagues were performing wind
tunnel tests on a new airfoil, under design within our division at
Grumman Aerospace. Somehow the subject of technical diving and buoyancy
compensation systems came up. We got to talking about the drag profiles
on certain manufacturers brands of BCs and one of the guys suggested that
we run some actual aerodynamics tests. A couple of the engineers are big
time mixed gas wreck divers with lots of experience on deep wrecks such
as the Doria and Republic. They were touting the benefits of the OMS
bungeed wings and I was arguing that the Dive Rite wings provided a
vastly improved knife edge profile with far less drag.

A simple surface area calculation showed that the bungeed wings had an
area of over 12 square feet inflated. This is over 4 times the area of
the Dive Rite wings. The huge box like profile caused immense drag and
horrendous turbulence when we observed it in the wind tunnel. The bungee
cords caused massive vorticial eddies all along the wings. It was very
interesting to observe the colored smoke trails streaming in chaotic
cyclonic patterns around the brightly colored bungees. It was almost
psychedelic. These results were in stark contrast to the Dive Rite wings
which showed very little turbulence and almost hydrodynamic behavior such
that the wings probably generate lift when there is current flow across
them. Needless to say, the extra inflator assembly on the OMS wings
created marked disturbances in the laminar flow field of the trace smoke.
When the wings were in the collapsed state the turbulent patterns were
even more spectacular, clearly discrediting the myth that the bungees
reduce drag by supposedly decreasing the surface area.

After the tests there were no more arguments except about what other uses
these wings might be put to. One of the guys suggested we employ them as
portable cushions for homeless people, another suggestion involved using
them as toxic waste storage containers.

There was much discussion and speculation about the personality and
physical characteristics of the rocket scientist who had designed these
wings. I suspect that the same individual carries a fire extinguisher in
the front seat of his car along with the spare gas can.

Rick

Taken from frogkick.dk, this is NOT my own work.

Man I wish we had pictures .... the OMS "Wings of Death" thread back in Tech diver would underscore this story nicely, and assure it a strong placement in the future article I plan to write, titled "Museum of Stupid".


Regards,
DanV

 

[D_B]

Not that I am a fan of bungied wings myself, but having both designed both ships hulls and aeroplane wings in my time... there is a huge difference between aerodynamics and hydrodynamics....!

..... why are they both called fluid dynamics?

Looks like the reason for a bungee'ed wing seams to be so you can run a larger than you need wing ... but why run one larger than you need?
The draw back is things on your wing adding drag and that they compress it making it .. harder to inflate by mouth, and more likely to exhaust all/most of your air out any small puncture

I notice that my SP Knighthawk wing outer edge is bungeed back on itself underneath so it folds the wing in half lengthwise, no outside bingee showing , and you can inflate it at least half way by mouth without expanding the bungees.. but it also has over 40lbs lift and again, why so much ? (I know that they have used it for doubles, maybe that was a reason?)

 

[Dr Wu]

So a bunch of engineers took considerable time and resources to perform this "test" but all we have as a result is this silly letter.

Where is the data? Where are the pictures?

Sounds like bull****.

 

[danvolker]

So a bunch of engineers took considerable time and resources to perform this "test" but all we have as a result is this silly letter.

Where is the data? Where are the pictures?

Sounds like bull****.

We also have several known deaths attributable to the bungeed wings...see thread Bungeed wings of death on techdiver or cavers....

A common theme in these deaths, was the bungeed wings having the bungees too tight, or having a dump valve issue which would not allow much pressure without dumping...and the eventuality here was that the bungede wings would not hold anywhere near the gas they should be holding for the expected inflation potential and lift they are supposed to have.

More over, the bungees "look cool" to a new diver..they "look" techy. But you do not need bungees to get the air out of a wing underwater
... the water will work well to allow you to vent all the gas from the wing you want to.
This is a MARKETING GIMMICK we know to have contributed to many past deaths, and this was covered ad nauseum on the tech list and cavers back around the early 1990 time frame.

Regards,
Dan

 

[HenrikBP]

I think that in order to draw any real conclusions a similar test would have to be performed in a water test facility with water flowing at speed relevant to divers ... i.e. slow.

The aerodynamic test is interesting, but probably only marginally relevant to diving situations.

In the interest of full disclosure I don't dive a bungeed wing

Henrik

 

[marinediva]

This is an extract from an article I read recently,

BAUE Wing Size FAQ

Bungied Wings

The use of bungied/bondage wings is strongly discouraged. To start with, one of the primary reasons stated for their use is that they streamline your rig. Ironically, they generally do the exact opposite. Hydrodynamics dictates that rough surfaces create increased turbulence which consequently increases drag. The bungies create a very rough surface and thus are adding to drag. Furthermore the bungies have a tendency to trap air which cause both static and dynamic instability issues. However these issues are not the most important reasons to avoid these wings. There are two large reasons that bungied wings are normally avoided. Probably the largest problem with bungied wings is the increased resistance to oral inflation. The bungies will make it significantly more difficult to orally inflate the wing, which can be a serious safety issue. A related issue is that the bungies create a stronger positive-pressure deflate than normally exists. So deflating the wing tends to dump air much faster than a non-bungied wing. This makes proper buoyancy control more difficult. And finally, due to this positive-pressure deflate, it is nearly impossible to use your BC as a "third regulator". This is somewhat of an advanced topic, however your BC can be used as an alternate regulator in very serious conditions. The deflate and inflate buttons are depressed simultaneously to provide air and then the diver breaths out their nose, or removes the BC inflator from their mouth when exhaling. It is important to note that you are not rebreathing the air in the BC, you are effectively breathing it straight from the BC inflator mechanism. Obviously you will want to practice this skill in a pool before trying it "for real"!

In relation to drag,
one of the first things we learn when diving is that when you need to slow yourself down if
experiencing a rapid ascent you spread yourself out to create drag.......
bigger surface space,
makes sense that a unit with bigger surface space will create more drag.

What would be more interesting to know,
is can a diver using a BP/w experience squat on the surface?

 

[D_B]

I think that in order to draw any real conclusions a similar test would have to be performed in a water test facility with water flowing at speed relevant to divers ... i.e. slow.

The aerodynamic test is interesting, but probably only marginally relevant to diving situations.

In the interest of full disclosure I don't dive a bungeed wing

Henrik

While I think your right about testing in water is going to be more accurate, any non laminar flow shown in a low speed wind tunnel, the eddies and turbulent flow, will be whats happening in the water at a much slower speed due to the density difference ... both air and water flow is called fluid dynamics because they behave the same